As shown in FIG. 1, a Venetian blind of the prior art comprises an upper rail 1, a lower rail 2, a plurality of cords 3, and a plurality of slats 4 which are held by the cords 3 between the upper rail 1 and the lower rail 2. The slats 4 are folded or unfolded by two pull cords in conjunction with an adjustment rod 5 which is used to actuate a modulating device 6 to set the slats 4 at any angle to regulate the light and the air passing through. The modulating device 6 is disposed in the upper rail 1. In order to prevent the adjustment rod 5 from interfering the slats 4, the worm rod 6a of the modulating device 6 is connected at one end of the adjustment rod 5 in such a way that the worm rod 6a is slantingly engaged with a worm gear 6b. As a result, the adjustment rod 5 and the slats 4 are separated by a distance.
The modulating device 6 of the prior art is defective in design in that the worm rod 6a is jutted out of the upper rail 1 such that the worm rod 6a is apt to interfere with the slats 4 located in proximity of the upper rail 1 at such time when the slats 4 are drawn up together to the top of the window. In addition, the modulating device 6 complicates the packaging of the Venetian blind. Moreover, the worm rod 6a is vulnerable to breakage in the course of transportation of the Venetian blind.